How to replace value for all keyframes?
If you mean you want to change its Z position throughout the animation, you can do it with the Graph Editor:


Select your object, open the Graph Editor.
Unfold the LocRotScale of the object and lock everything except the Z Location.
Select the curve and move it up 2 units with G, Y and 2 (Y is the up axis of the Graph Editor).
Now your object is 2 units up on the Z axis.




Now, about your second question: "If some of my z positions are not 1, but a different value, is there a way to set them all at once, rather than shift them all by the same value?"

Trick to align several vertices on the same axis:


Select the vertice that is your Y reference, and ctrl G to move your green cursor up to this vertice
Select the vertices you want to align on the same Y position.
shift S > Cursor Value.


Better trick suggested by Mr Zak:


Select the vertice #1 which is the Y position reference.
ctrl G to make the cursor move to the vertice position.
Select the 2D Cursor as the Pivot Point.
Select all the vertices you want to snap to the Y position.
Scale down to zero with S Y 0